1. Field of the Invention
This invention relates generally to a system and method for controlling airflow to a fuel cell stack in the event of a cathode input flow meter failure and, more particularly, to a system and method for controlling the flow of cathode input air to a fuel cell stack in a fuel cell system in the event that a flow meter for measuring the airflow fails by providing open-loop control of a compressor.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
Proper airflow measurement and control to the cathode side of a fuel cell stack is critical for the operation of a fuel cell system. If too much air is delivered to the stack, energy is wasted and the fuel cells in the stack may become too dry, affecting their durability. Too little air delivered to the stack can result in fuel cell instability due to oxygen starvation. Therefore, fuel cell systems typically employ an airflow meter in the cathode input line to provide an accurate measurement of the flow of air to the fuel cell stack. If the airflow meter fails, it has typically been necessary to shut the fuel cell system down because by not knowing the amount of air being delivered to the fuel cell stack with enough accuracy could have a detrimental effect on system components.
In order to increase the reliability of a fuel cell system, it is desirable to continue to operate the system in the event that the primary cathode airflow measuring device fails and to maintain an acceptable level of performance without causing long term damage to the system or stack components.